Re: [PATCH bpf-next v1 0/3] add bpf_file_d_path helper and selftests

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/18/24 4:51 AM, Lin Yikai wrote:
> v1:
>   - patch 2:
>     - [1/2] add bpf_file_d_path helper
>     - [2/2] add selftest to it
> 
> Hi, we are looking to add the "bpf_file_d_path" helper,
> used to retrieve the path from a struct file object.
> 	bpf_file_d_path(void *file, char *dst, u32 size);
> 	
> It's worth noting that the "file" parameter is defined as "void*" type.
> 
> * Our problems *
> Previously, we encountered issues
> on some user-space operating systems(OS):
> 
> 1.Difficulty using vmlinux.h
> (1) The OS lacks support for bpftool.
> We can not use:
> "bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h".
> Bpftool need a separate complex cross-compilation environment to build.
> 
> (2) Many duplicate definitions between OS and vmlinux.h.
> 
> (3) The vmlinux.h size is large (2.8MB on arm64/Android),
> causing increased ebpf prog size and user space consumption.

The compiled bpf prog size is increased by 2.8MB because it included vmlinux.h?

> 
> 2.The "struct file" has many internal variables and definitions,
> and maybe change along with Linux version iterations,
> making it hard to copy it to OS.

If vmlinux.h is not convenience in your use case, you can try to define "struct 
file" with __attribute__((preserve_access_index)) and the libbpf will adjust the 
bpf prog against the running kernel.

There was a discussion a year ago about bpf helpers freeze. No new helper can be 
added since then. The same goes for this one.

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